This invention relates to a fluidized bed reactor and, more particularly, to a fluidized bed reactor utilizing a water cooled internal solids separator.
Reactors, such as combustors, steam generators and the like, utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at relatively low temperatures. When the combustor is utilized as a steam generator, the heat produced by the fluidized bed is utilized to convert water to steam which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a "bubbling" fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. The gas velocity is typically two to three times that needed to develop a pressure drop which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give the appearance of a boiling liquid.
In a steam generator environment, the walls enclosing the bubbling bed are formed by a plurality of heat transfer tubes, and the heat produced by combustion within the fluidized bed is transferred to water circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a stream drum, for separating water from the steam thus formed which is routed to a turbine or to another steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed. In these arrangements the mean gas velocity is increased above that for the bubbling bed, so that the bed surface becomes more diffused and the solids entrainment from the bed is increased. According to this process, fluidized bed densities are attained which are well below those typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed is due to its small particle size and to a high solids throughput, which requires high solids recycle. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity beyond which the bed would be converted into a pneumatic transport line.
In both a bubbling fluidized bed and a circulating fluidized bed, the mixture of air and gaseous products of combustion leaving the bed entrains solid particles from the bed. In most applications, these entrained particles must be separated from the gases and recycled back into the bed, requiring the use of externally disposed units, usually in the form of cyclone separators. However, these separators are relatively expensive, take up a large space and are susceptible to mechanical failure due to the high temperature environment.